1. Field of the Invention
This invention relates to sodium-sulfur storage batteries and particularly to a novel sulfur electrode for such a battery.
2. Description of Prior Art
The conventional sodium-sulfur secondary battery comprises a casing containing sodium and sulfur reactants, the reactants being molten at the operating temperature of the battery, separated by a solid electrolyte, such as beta-alumina ceramic. During discharge, the sodium is ionized and these ions pass through the solid electrolyte and react with sulfur in the sulfur electrode which conventionally comprises sulfur that is molten at the battery operating temperature and a current collector, to form sodium polysulfide (Na.sub.2 S.sub.x). The energy generated by this reaction is electrically removed from the battery through terminals provided therein. The battery may be recharged by supplying current thereto from an external source in the reverse direction, causing the sodium polysulfide produced during discharge to decompose and pass as sodium ions through the solid electrolyte to the molten sodium region of the cell where metallic sodium is formed by electron addition.
It is important in a battery of this type that the current collector portion of the sulfur electrode has a high current collecting ability, and it is required that the structure has high strength and excellent corrosion resistance. Additionally, good sodium-sulfur battery performance depends to a large extent on the distribution of sulfur in the current collector.
It is well known in the prior art to utilize carbon or graphite felt or yarn in various forms as the current collector portion of the sulfur electrode for a sodium-sulfur battery. U.S. Pat. No. 3,883,367--Chiku et al., for example, discloses a sodium-sulfur battery having a porous current collector formed of a knitted or woven carbon yarn as part of its sulfur electrode. Sulfur is absorbed on the felt or yarn and wicking is the dominant mode for mass transport of reactants in the sulfur electrode during operation thereof. Acceptable performance in this type of battery requires a continuous generally uniform distribution of high porosity carbon or graphite throughout the sulfur electrode, and the amount of absorbance of the reactants by the felt or yarn essentially depends on the available surface area per unit volume of absorbent.
While known current collectors for sulfur electrodes in sodium-sulfur batteries partially fulfill the requirements presented above, several problems remain unsolved. It is obvious that ideal sulfur distribution cannot be obtained in current collectors known in the art, in view of the methods of adding the molten sulfur to the collector. Further, the carbon or graphite felt or yarn conventionally utilized in the current collector cannot be uniformly distributed in the battery due to the flexibility of this material. The material must be pressed against the solid electrolyte by a backing plate to assure adequate electrical contact thereto. This pressure restricts the flow of Na.sub.2 S.sub.x through the current collector. Additionally, due to the physical characteristics of the material, it is not possible to machine the current collector to required specifications. When a knitted or woven carbon or graphite fiber shape is used, a further problem is that the separate fibers are not bonded to each other, resulting in increased resistivity of the sulfur electrode.